1. Field of the Invention
The invention relates to a magnetic resonance examination apparatus and method of operation thereof in which two 90.degree. rf pulses are generated in the presence of a magnetic gradient field in order to generate a selective 180.degree. rf pulse.
2. Description of the Related Art
A magnetic resonance examination method of this kind is known from EP-OS 143 602. The known method concerns a spin echo sequence in which, after a 90.degree. rf pulse, two 90.degree. rf pulses are generated, at a short distance in time from one another, in the presence of a magnetic gradient field, which 90.degree. rf pulses serve to act together as one 180.degree. rf pulse.
A selective rf pulse is to be understood to mean herein an rf pulse which influences the nuclear magnetization only in a part of the space exposed to uniform, steady magnetic field (always required during magnetic resonance examinations) and the field of the rf coil generating the rf pulses. A 90.degree. rf pulse and a 180.degree. rf pulse are to be understood to mean an rf pulse which rotates the nuclear magnetization, or vector components thereof, through 90.degree. and 180.degree., respectively.
The generating of two closely successive selective 90.degree. rf pulses instead of a single selective 180.degree. rf pulse offers various advantages. On the one hand, the product of power and bandwidth required is thus cut in half; on the other hand, it is substantially simpler to determine the variation in time of a selective 90.degree. rf pulse than of a selective 180.degree. rf pulse. This is because for small flip angles (i.e. the angle wherethrough the nuclear magnetization is rotated with respect to the direction of the uniform, steady magnetic field) a substantially linear relationship exists between the spectrum of the rf pulse and the profile of the layer to be excited by this rf pulse. The variation in time of an rf pulse can thus be obtained by means of a simple Fourier transformation of the pulse spectrum which is proportional to the layer profile. However, this is applicable only to flip angles up to approximately 90.degree.. This method fails for flip angles of 180.degree. ; in that case exact solution of the Block equations is required. Variations in time of the rf pulse may then occur which can be generated only by means of complex technical steps.
The known method also has a further drawback in that the magnitude of the nuclear magnetization in the selectively excited region is also determined by the homogeneity of the steady magnetic field. This may be drawback notably for inversion recovery sequences where first a selective 180.degree. rf pulse is generated.